Investigators on Monday worked to uncover what ignited a fire at an abandoned home in Fresno that trapped a group of squatters, killing five people and critically injuring another who tried to flee out the only open window.

Firefighters found the six victims in one of the bedrooms, where it appeared they were trying to smash a window to escape the flames and thick smoke engulfing the boarded-up home on Sunday.

One of the sealed windows had been pried open, and that was the only way to get in or out of the house, Battalion Chief Lawrence French said. That's also where the fire was the most intense, he said.

A neighbor told police he heard people screaming for help and pounding against a window, French said.

Four died at the scene, and the other died at a hospital. The injured person was reported to be in critical condition at Community Regional Medical Center, according to the Fresno Bee.